The invention relates to a process and apparatus for arc welding of a metallic component to a metallic structure. More particularly, the invention relates to the welding of a metallic stud or nut to a metallic sheet on which stud or nut is to be used as an anchorage.
The welding of metallic studs and nuts to a metallic structure by means of arc welding is a widespread process found in particular application in the automotive industry. The exact method of the welding process can be relatively varied but the general principal involves the formation of an electric arc between the stud and the metal sheet effecting relative melt of the metallic materials in the region of the arc so that the metal component can then be lowered into the melt which is formed in the fusion region to result in a strong welded connection being formed between the component and the metal structure on subsequent cooling.
One form of welding process is the so called drawn arc welding process which involves positioning the component to be welded to the metal sheet within a weld gun and bringing the component into electrical contact with the metallic surface to complete an initial circuit indicating that the metal component is in contact with the metal sheet. The weld cycle is then initiated whereby the stud is withdrawn from the work surface inducing a pilot arc which serves to help clean both the component and the work surface before a main welding current is initiated creating a weld arc between the raised component and the metal sheet. The weld arc serves to form a pool of molten metal on the sheet material and also on the metallic component. The welding apparatus then causes the component to be plunged into the molten metal whereby as the weld pool solidifies it forms a homogenous joint. The entire process takes less than a second and forms a joint which in fact is stronger than material that has actually been welded. Such a drawn arc welding process is standard within the welding industry and is used to attach both studs and nuts to metal sheets which allow further fastenings to be connected thereto. In this manner studs themselves may often be threaded to receive a co-operating threaded nut or a nut itself may be welded directly onto the sheet in order to receive a screw threaded connector. Usually such studs and nuts are to enable earthing connections to be made to the sheet metal and thus require good electric contact to be made between the stud or nut and the appropriate connector element fixed thereto. However, it has been found that even with the cleaning function of the pilot arc, the intense heat generated by the arc during the welding process results in the formation of vaporised carbon and other impurities which can result in a xe2x80x9csmokexe2x80x9d emanating from the molten material. Condensation of this smoke and vaporised impurity on the threads above the welded nut or stud can serve to inhibit electrical contact with the subsequent connection and impair the threaded engagement between the component and threaded connector.
Furthermore, it has been found that due to the reactive forces generated by the formation of the arc itself and also by the driving of the component into the molten metal can result in the formation of splashes of molten metal whereby if such splashes land on the thread of the nut or the stud, then they again will solidify to inhibit the screw threaded operation of such nut or stud. It is therefore an object of the present invention to provide for an improved welding process and welding apparatus to alleviate the aforementioned problems in a manner which will not inhibit the speed or quality of the welding procedure.
According to one aspect of the present invention there is provided a method of welding a substantially metal component to a metal workpiece, comprising the steps of: creating a weld arc between said component and metal workpiece to create a weld pool of molten material and initiating a pressurised fluid flow over said component and the weld pool to deflect any airborne fluid residue of the weld pool away from said component during the displacement of the component into the weld pool.
Preferably, where the method is applicable to a component which comprises a metal weld region and a connector region, the weld arc is created between the metal weld region of the component and the metal workpiece to create a weld pool and the pressurised fluid flow is initiated so as to be disposed between the connector region of the component and the weld pool to deflect the fluid residue of the weld pool from the connector region. Usually the flow of pressurised fluid is initiated as least substantially simultaneously with the creation of the weld arc or before creation of the weld arc.
Where the connector region is threaded, the method will usually comprise the step of deflecting the fluid flow through a channel formed by the thread of the threaded region. In particular, where the component is a weld nut the pressurised fluid flow is directed through the central aperture of the nut to create a positive pressure within the aperture of the nut which prevent any splashes of molten weld material or gases from entering this region. The method will also preferably comprise the step of placing a solid cylindrical insert in frictional engagement with the internal threads of the nut in order to form sealed channels with the thread through which the fluid flow can then be directed. In this manner the pressurised fluid is further compressed through this threaded channel to create a high positive pressure which prevents any airborne particulate from the weld procedure from being thrown or drawn into the threaded region of the nut.
The pressurised fluid may be given a straight flow path prior to being directed through the nut. This flow path can be induced by passing the fluid through at least one longitudinally extending channel, usually formed within a solid control valve placed in the fluid path, and more usually the flow is induced by passing through six such channels, which are equally spaced angularly about the central axis of the fluid flow path.
The pressurised fluid may also be given a spiral flow path prior to being directed through the nut. Thus when the fluid flow engages with the spiral threads of the nut it has already partially aligned with such threads and therefore readily engages and enters the channels formed between the threads of the nut. This spiral flow path is preferably induced by passing the fluid through at least one spiral channel, usually formed within a solid control valve placed in the fluid path, and more usually the spiral flow is induced by passing through six such channels, which are equally spaced angularly about the central axis of the fluid flow path. The spiral channels are usually set to deflect the fluid flow at an angle of between 10 and 80xc2x0 relative to a plane perpendicular to the direction of fluid flow and more particularly the angle of deflection generated is between 50 and 70xc2x0.
The pressurized fluid may also b allowed to expand after the spiral flow path is induced therein. Preferably, the fluid use in this method will be compressed air directed from a pressurized air source at a flow ate of between 5 and 30 liters per minute and preferably between 10 and 20 liters per minute.
Furthermore, according to a second aspect of the present invention there is provided apparatus for directing a pressurised fluid flow over a substantially metal component during arc welding, the fluid flow being directed by said apparatus so as to deflect any airborne fluid residue created by the welding process away from the component.
According to a further aspect of the present invention there is also provided welding apparatus for welding a substantially metal component to a metal workpiece, said component comprising a metal weld region and a connector region, said apparatus comprising a weld head having means for holding said weld region of said component adjacent said workpiece and inducing a weld arc between said weld region and workpiece to create a weld pool of molten material, whereby the weld head subsequently displaces said weld region into said weld pool; said apparatus further comprising a pressurised fluid source and a fluid flow control means for directing said pressurised fluid over said component and the weld pool so as to deflect any airborne fluid residue of the weld pool away from said connector region of said component.
Usually such apparatus will comprise control means for initiating fluid flow at least substantially simultaneously with or before the creation of the weld arc. Such control means will usually be by means of a computerised control station as are conventional for arc welding apparatus (both automated and hand held) whereby such control unit will detect, by means of a test voltage, when welding should occur and thereby automatically directs a weld current to the weld head to create the weld arc. Such a computerised control unit can, simultaneously or slightly prior to inducing the weld current, initiate fluid flow from the fluid source in a conventional manner such as opening a valve or any other conventional method of transmitting a pressurised fluid from a pressurised fluid source. This computerised control unit will also control displacement of the weld head and nozzle.
The welding apparatus will usually be used with a component having a threaded connector region whereby the fluid control means will comprise a directing member on the outer edges of the threads so as to form a substantially sealed channel therewith and to then direct such fluid flow through this thread channel. With an externally threaded member such as a screw threaded stud such a directing member may form a tubular body which slides over the exterior surface of the stud so as to engage such threads, but in the case of a threaded nut then the outer edges of the threads are directed about the internal periphery of the internal nut aperture in which case a cylindrical body is inserted into the nut so as to engage these threads.
The fluid flow control means will usually comprise a substantially hollow fluid transmitting member with an end stop for substantially forming a sealed engagement with the upper surface of a weld nut so that fluid flow is directed through the central aperture of such nut. Such fluid flow will then be prevented from continued displacement through such nut where the cylindrical body has been inserted and thus is forced into the channel formed between the thread of the nut and the cylindrical body to pass therethrough. This provides the advantage of specifically directing the pressurised fluid flow through the threads of the nut and also serves to increase the pressure of the fluid through these channels thereby preventing any airborne residual particles from the weld pool from entering by the nut thread.
The welding apparatus will also preferably comprise a control valve disposed between the fluid source and the nut for inducing a flow path in the fluid flow. This control valve will usually comprise a solid body having at least one channel extending therethrough. The control valve will be cylindrical and may have six channels formed about its periphery so as to be equally spaced angularly about the axis of such valve. The channels may by spiral and, if so, will usually be formed at an angle of between 10 and 80xc2x0 to a plane perpendicular to the direction of flow.
In addition, a deflector may be disposed in the fluid flow in front of the control valve and angularly inclined in the direction of fluid flow to gradually deflect fluid flow towards the entrances to the spiral channels. This is to alleviate the formation of eddy currents whereby the pressurised fluid is forced through the smaller apertures of the spiral channels which could disrupt formation of the spiral fluid flow. Usually such a deflecting member comprises a cone.
In addition, where the fluid flow has been compressed through channels in a fluid control valve the fluid control means may comprise an expansion chamber in which the fluid may enter after passing through the fluid control valve to allow expansion of the pressurised fluid after spiral flow is induced. This expansion of the pressurised fluid enables the spiral flow path to be retained but reduces the pressure slightly after having been compressed through the channels in the control valve so that it more readily engages with and enters into the channels formed between the threaded region of the nut and the cylinder placed therein.
This welding apparatus is highly applicable to existing welding apparatus whereby the leading arm of conventional weld heads can be utilised to form a substantially airtight cylinder through which the fluid flow may pass and thus be directed to the component during the weld operation whereby the weld control valve can be simply incorporated within the end of such a member 14.